Vapor deposition process and apparatus therefor

ABSTRACT

An apparatus for depositing a ceramic coating on a component. The apparatus is configured to make use of an evaporation source containing multiple different oxide compounds, in which at least one of the oxide compounds has a vapor pressure that is higher than the remaining oxide compounds. The apparatus is operable to introduce the evaporation source into a coating chamber, suspend the component near the evaporation source, and project a high-energy beam on the evaporation source to melt and form a vapor cloud having a composition comprising the oxide compounds of the evaporation source. The apparatus includes a feature that prevents the vapor cloud from contacting and condensing on the component during an initial phase of operation, and subsequently permit and then again prevent the vapor cloud from contacting and condensing on the component during subsequent phases of operation in response to changes in the composition of the vapor cloud.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation patent application of co-pending U.S. patentapplication Ser. No. 10/709,668 filed May 21, 2004, which is a divisionpatent application of prior co-pending application Ser. No. 10/064,887filed Aug. 27, 2002, now U.S. Pat. No. 6,790,486. The contents of theseapplications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to coating processes. Moreparticularly, this invention is directed to a vapor deposition processand apparatus for depositing ceramic coatings containing multiple oxideswith different vapor pressures using a single evaporation sourcecontaining the multiple oxides.

Higher operating temperatures for gas turbine engines are continuouslysought in order to increase their efficiency. However, as operatingtemperatures increase, the high temperature durability of the componentswithin the hot gas path of the engine must correspondingly increase.Significant advances in high temperature capabilities have been achievedthrough the formulation of nickel and cobalt-base superalloys.Nonetheless, certain components of the turbine, combustor and augmentorsections of a gas turbine engine can be required to operate attemperatures at which the mechanical properties of such alloys would beinsufficient. For this reason, these components are often protected by athermal barrier coating (TBC) formed of a ceramic material. Because ofthe different coefficients of thermal expansion (CTE) between ceramicmaterials and the superalloy substrates they protect, anoxidation-resistant bond coat is typically employed to promote adhesionand extend the service life of a TBC, as well as protect the underlyingsubstrate from damage by oxidation and hot corrosion attack. Bond coatsused on superalloy substrates are typically in the form of an overlaycoating such as MCrAIX (where M is iron, cobalt and/or nickel, and X isyttrium or another rare earth element), or a diffusional uminidecoating. During the deposition of the ceramic TBC and subsequentexposures to high temperatures, such as during engine operation, thesebond coats form a tightly adherent alumina (Al₂O₃) layer or scale thatadheres the TBC to the bond coat.

Various ceramic materials have been proposed for TBC's, the most notableof which is zirconia (ZrO₂) that is partially or fully stabilized byyttria (Y₂O₃), magnesia (MgO) or another alkaline-earth metal oxides, orceria (CeO₂) or another rare-earth metal oxides. Binaryyttria-stabilized zirconia (YSZ) is widely used as a TBC materialbecause of its high temperature capability, low thermal conductivity anderosion resistance in comparison to zirconia stabilized by other oxides.YSZ is also preferred as a result of the relative ease with which it canbe deposited by plasma spraying, flame spraying and physical vapordeposition (PVD) techniques. TBC's employed in the highest temperatureregions of gas turbine engines are often deposited by PVD, particularlyelectron beam physical vapor deposition (EBPVD), which yields acolumnar, strain-tolerant grain structure that is able to expand andcontract without causing damaging stresses that lead to spallation.Similar columnar microstructures can be produced using other atomic andmolecular vapor processes, such as sputtering (e.g., high and lowpressure, standard or collimated plume), ion plasma deposition, and allforms of melting and evaporation deposition processes (e.g., cathodicarc, laser melting, etc.).

In order for a TBC to remain effective throughout the planned life cycleof the component it protects, it is important that the TBC has andmaintains a low thermal conductivity. However, the thermalconductivities of TBC materials such as YSZ are known to increase overtime when subjected to the operating environment of a gas turbineengine. As a result, TBC's for gas turbine engine components are oftendeposited to a greater thickness than would otherwise be necessary.Alternatively, internally cooled components such as blades and nozzlesmust be designed to have higher cooling flow. Both of these solutionsare undesirable for reasons relating to cost, component life and engineefficiency. As a result, it can be appreciated that further improvementsin TBC technology are desirable, particularly as TBC's are employed tothermally insulate components intended for more demanding enginedesigns.

To reduce and stabilize the thermal conductivity of YSZ, ternary YSZsystems have been proposed. For example, commonly-assigned U.S. Pat. No.6,586,115 to Rigney et al. discloses a TBC of YSZ alloyed to containcertain amounts of one or more alkaline-earth metal oxides (magnesia(MgO), calcia (CaO), strontia (SrO) and barium oxide (BaO)), rare-earthmetal oxides (lanthana (La₂O₃), ceria (CeO₂), neodymia (Nd₂O₃),gadolinium oxide (Gd₂O₃) and dysprosia (Dy₂O₃)), and/or such metaloxides as nickel oxide (NiO), ferric oxide (Fe₂O₃), cobaltous oxide(CoO), and scandium oxide (Sc₂O₃). According to Rigney et al., whenpresent in sufficient amounts these oxides are able to significantlyreduce the thermal conductivity of YSZ by increasing crystallographicdefects and/or lattice strains. In commonly-assigned U.S. Pat. No.6,808,799 to Darolia et al., a TBC of YSZ is deposited to contain athird oxide, elemental carbon and potentially carbides and/or acarbon-containing gas. The resulting TBC is characterized by lowerdensity and thermal conductivity, high temperature stability andimproved mechanical properties.

While the incorporation of additional oxide compounds into a YSZ TBC inaccordance with Rigney et al. and Darolia et al. has made possible amore stabilized TBC microstructures, it can be difficult to deposit aTBC by an evaporation process to produce a desired and uniformcomposition if the additional oxide has a significantly different vaporpressure than zirconia and yttria. For example, ceria has a vaporpressure of about 10 mbar, in comparison to vapor pressures of about0.05 mbar for zirconia and yttria at 2500° C. If a YSZ+ceria TBC is tobe deposited by EBPVD or another vapor deposition process, evaporating asingle ingot containing the desired YSZ+ceria composition deposits a TBCthat has an unacceptable nonuniform distribution of ceria. To avoid thisresult, co-evaporation of oxides having vapor pressures significantlydifferent from YSZ (e.g., at least an order of magnitude higher thanYSZ) has been performed with a separate ingot of each additional oxide.If a single electron beam is used, a controlled beam jumping techniquemust be employed, by which the beam is briefly projected (in themillisecond range) on each ingot, with the amount of time on each ingotbeing adjusted so that the energy output achieves the energy balancerequired to obtain compositional control. As an alternative to the useof a single beam, multiple electron guns operated at different powerlevels have been used to maintain molten pools of each ingot material.However, both of these techniques complicate the deposition process suchthat the incorporation of additional oxides in a YSZ TBC can bedifficult to perform in a commercial setting.

In view of the above, it would be desirable if a process existed thatsimplified the co-evaporation of oxides with different vapor pressures.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a process and apparatus for depositing aceramic coating, such as a thermal barrier coating (TBC) for a componentintended for use in a hostile thermal environment, such as thesuperalloy turbine, combustor and augmentor components of a gas turbineengine. The process and apparatus of this invention are particularlydirected to an evaporation technique for depositing a TBC formed ofmultiple oxide compounds, at least one of which has a vapor pressurethat differs from the remaining oxide compounds. An example is in thedeposition of a TBC formed of YSZ alloyed with a third oxide to reducethe density and/or thermal conductivity of the TBC, improve hightemperature stability, and/or improve mechanical properties.

The apparatus is configured to make use of a single evaporation sourcecontaining multiple different oxide compounds, in which at least one ofthe oxide compounds has a vapor pressure that is higher than theremaining oxide compounds. The apparatus is further configured tointroduce the evaporation source into a coating chamber, suspend thecomponent near the evaporation source, and project a high-energy beam onthe evaporation source to melt and form a vapor cloud having acomposition comprising the oxide compounds of the evaporation source.The apparatus includes a feature that prevents the vapor cloud fromcontacting and condensing on the component during an initial phase inwhich the composition of the vapor cloud is such that the relativeamount of the at least one oxide compound in the vapor cloud is greaterthan the relative amount of the at least one oxide compound in theevaporation source. The apparatus is responsive to changes in thecomposition of the vapor cloud so as to remove the preventing featureand allow the vapor cloud to contact and condense on the component toform the coating during a subsequent phase in which the composition ofthe vapor cloud has changed so that the relative amount of the at leastone oxide compound in the vapor cloud is approximately equal to therelative amount of the at least one oxide compound in the evaporationsource. The apparatus is operable to position the preventing featurebetween the evaporation source and the component following thesubsequent phase and during a second subsequent phase in which aremaining portion of the evaporation source is relatively rich in theremaining oxide compounds.

The process generally entails providing an evaporation source containingmultiple different oxide compounds, at least one of the oxide compoundshaving a vapor pressure that is higher than the remaining oxidecompounds. In a YSZ coating system, examples of particularly suitableoxide compounds are metal oxides of metals such as cerium, gadolinium,neodymium, lanthanum, dysprosium, ytterbium, tantalum, magnesium,calcium, strontium and barium, which have a sufficient absolute percention size difference relative to zirconium ions to produce significantlattice strains that promote lower thermal conductivities. The componentintended to be coated is suspended near the evaporation source, and ahigh-energy (e.g., electron or laser) beam is projected onto theevaporation source to melt and form a vapor cloud of the oxide compoundsof the evaporation source, while preventing the vapor cloud fromcontacting and condensing on the component during an initial phase inwhich the relative amount of the one oxide compound in the vapor cloudis greater than the relative amount of the oxide compound in theevaporation source. For this purpose, a barrier may be physically placedbetween the component and the evaporation source. During a subsequentphase, in which the relative amount of the oxide compound in the vaporcloud has decreased to something approximately equal to its relativeamount in the evaporation source, the vapor cloud is allowed to contactand condense on the component to form the coating. If a barrier was usedto initially prevent deposition of the coating, the barrier is removedduring this subsequent phase of the evaporation process.

In view of the above, it can be appreciated that the present inventionis based on a determination that, at the beginning of an evaporationprocess using an evaporation source (e.g., ingot) containing multipleoxide compounds including one or more whose vapor pressure is higherthan the other oxide compounds, the vapor cloud is rich with the oxidecompound with the highest vapor pressure, as a result of the oxidecompound evaporating faster than the other oxide compounds. Furthermore,it was determined that over a period of time, the evaporation sourcebecomes enriched in the oxide component(s) having lower vapor pressures(corresponding to lower evaporation rates), with the result that anequilibrium (or near equilibrium) is established in the evaporationprocess, resulting in a more uniform co-evaporation of the oxidecompounds in the evaporation source. As a result, a coating depositedduring this phase of the evaporation process will have a compositionmore nearly equal to that of the evaporation source. Accordingly, apreferred aspect of the present invention is to allow the vapor cloudevaporated from an evaporation source to contact and condense on thearticle primarily or exclusively during this later phase, producing acoating whose composition is more predictable and uniform than otherwisepossible when using a single evaporation source for the multiple oxidecompounds.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an EBPVD apparatus using asingle evaporation source containing multiple oxide compounds, one ofwhich has a significantly higher vapor pressure than the remaining oxidecompounds of the source in accordance with one embodiment of the presentinvention.

FIG. 2 is a graph representing the concentration in a coating of theoxide compound with the higher vapor pressure, plotted relative to timeduring the coating process.

FIG. 3 is a microphotograph of a cross-section through a thermal barriercoating deposited in accordance with the present invention.

FIG. 4 is a graph of the chemical composition of a thermal barriercoating deposited in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally applicable to components subjected tohigh temperatures, such as the high and low pressure turbine nozzles andblades, shrouds, combustor liners and augmentor hardware of gas turbineengines. While the advantages of this invention are particularlyapplicable to gas turbine engine components, the teachings of thisinvention are generally applicable to any component on which a TBC maybe used to provide protection from a high temperature environment.

TBC's of particular interest to the invention are typically bonded to asubstrate, such as a superalloy material, with a metallic bond coatdeposited on the substrate. The bond coat is preferably an aluminum-richcomposition, such as an overlay coating of an MCrAlX alloy or adiffusion coating such as a diffusion aluminide or a diffusion platinumaluminide of a type known in the art, though it is foreseeable thatother bond coat materials and types could be used. As with prior artTBC's, TBC's of this invention are intended to be deposited to athickness that is sufficient to provide the required thermal protectionfor the particular component, typically on the order of about 75 toabout 300 micrometers, though lesser and greater thicknesses areforeseeable.

To achieve a strain-tolerant grain structure, TBC's are deposited usinga physical vapor deposition technique, such as EBPVD, though otherevaporation techniques are possible and within the scope of thisinvention. The EBPVD process requires the presence of an evaporationsource formed essentially of the coating composition desired, and anelectron beam at an appropriate power level to create a vapor of theevaporation source in the presence of the surface to be coated. FIG. 1schematically represents a portion of an EBPVD coating apparatus 20,including a coating chamber 22 in which a component 30 is suspended forcoating. A TBC 32 is represented as being deposited on the component 30by melting and vaporizing an ingot 10 of the desired coating materialwith an electron beam 26 produced by an electron beam gun 28. Theintensity of the beam 26 is sufficient to produce a vapor cloud 34 thatcontacts and then condenses on the component 30 to form the TBC 32. Asshown, the vapor cloud 34 evaporates from a pool 14 of molten coatingmaterial contained within a reservoir formed by a crucible 12 thatsurrounds the upper end of the ingot 10. Water or another suitablecooling medium flows through cooling passages 16 defined within thecrucible 12 to maintain the crucible 12 at an acceptable temperature. Asit is gradually consumed by the deposition process, the ingot 10 isincrementally fed into the chamber 22 through an airlock 24. As a resultof the vapor process by which the TBC 32 is deposited, the individualgrains of the TBC 32 are characterized by microstructural defects andpores within the grains and at and between the grain boundaries. Thesedefects and pores are believed to decrease the thermal conductivity ofthe individual TBC grains, and therefore the TBC 32 as a whole.

According to a preferred aspect of the invention, the thermal-insulatingmaterial of the TBC 26 is based on binary yttria-stabilized zirconia(YSZ), but alloyed to contain at least a third metal oxide. Theinvention particularly pertains to the deposition by evaporation ofYSZ-based coatings in which one or more of the additional metal oxideshave a vapor pressure that differs significantly from zirconia andyttria, e.g., at least an order of magnitude. Though not a necessaryfeature of the invention, the third oxide preferably has the effect ofreducing and/or stabilizing the thermal conductivity of the TBC 32. Forthis purpose, and in accordance with commonly-assigned U.S. Pat. No.6,586,115 to Rigney et al., the third oxide preferably has an absolutepercent ion size difference relative to zirconium ions of at least thatof an yttrium anion (Y³⁺), i.e., at least 13 percent, so as to producesignificant strains due to ionic size. In accordance withcommonly-assigned U.S. Pat. No. 6,808,799 to Darolia et al., the TBC 32may be further modified to contain elemental carbon in the form ofprecipitate clusters, from which may evolve a carbon-containing gas(e.g., carbon monoxide (CO) and/or carbon dioxide (CO₂)) as a result ofthermal decomposition of carbon. In combination, the presence ofelemental carbon clusters and one or more of the above-specified thirdmetal oxides is believed to reduce the density and thermal conductivityof a YSZ TBC.

The TBC 32 preferred for this invention preferably contains about 3 toabout 8 weight percent yttria, though lesser or greater amounts ofyttria could be used. Examples of suitable oxide compounds to be alloyedwith YSZ are metal oxides such as cerium, gadolinium, neodymium,lanthanum, dysprosium, ytterbium, tantalum, magnesium, calcium,strontium and barium, which have vapor pressures that differsignificantly from zirconia and yttria. For example, ceria, neodymia,lanthana, ytterbia, magnesia, strontia and barium oxide are all believedto have vapor pressures higher than zirconia and yttria, some more thanan order of magnitude higher. Based on the teachings of Rigney et al.and Darolia et al., ceria in amounts of about 10 to about 20 weightpercent of the TBC 32 is believed to be particularly beneficial, thoughit is foreseeable that lower and higher levels of ceria could be used.

According to the present invention, YSZ and ceria (or another high vaporpressure oxide) are simultaneously evaporated from a single ingot 10having the desired composition for the coating 32, even though prior artattempts to co-evaporate YSZ and ceria have produced coatings whosecompositions are not uniform or consistent with the composition of theingot as a result of the higher vapor pressure of ceria. In aninvestigation leading to this invention, an ingot containing YSZ alloyedwith about 16 weight percent ceria was evaporated by EBPVD to deposit aTBC. With reference to FIG. 2, analysis of the coating indicated thatthe portion of the TBC deposited at the beginning of the evaporationprocess (t₁) was rich in ceria. The ceria content dropped through theinitial thickness of the TBC, corresponding to a coating duration ofabout ten minutes, after which the ceria content was relatively stablewithin the coating (t₂) before dropping off near the end of the coatingoperation (t₃). From this investigation, it was concluded that the vaporcloud 34 within the coating chamber is initially rich in ceria ions as aresult of the higher vapor pressure of ceria (corresponding to a higherevaporation rate). However, after a period of time (t₁) an equilibrium(or near equilibrium) appeared to become established in the evaporationprocess, resulting in a more uniform co-evaporation of YSZ and ceriafrom the ingot. While not wishing to be held to any particular theory,it was concluded that the ingot had become sufficiently enriched in YSZas a result of the lower evaporation rates of yttria and zirconia(resulting from their relatively lower vapor pressures), that theapparent equilibrium was established for the evaporation rates ofzirconia, yttria and ceria. The final drop-off of the ceria content inthe coating (t₃) was attributed to the remainder of the ingot being richin YSZ from the earlier accelerated lose of ceria without any additionalceria available from the bulk of the ingot.

On this basis, it was concluded that a TBC 32 deposited during theintermediate phase (t₂) of the evaporation process can have acomposition more nearly equal to that of the ingot. Accordingly, anobject of the invention is to allow the vapor cloud 34 evaporated fromthe ingot 10 to contact and condense on the component 30 primarily orexclusively during this intermediate phase, to produce a TBC 32 whosecomposition is more predictable and uniform than otherwise possible whenusing a single evaporation source. With reference again to FIG. 1, theEBPVD apparatus 20 is depicted as including a barrier 36 that is shownto be positioned between the component 30 and the molten pool 14,representative of the initial or latter phases of the coating process inwhich the proportional composition of the vapor cloud 34 differs fromthe ingot 10. A suitable barrier 36 is a stainless steel plate that canbe maneuvered from outside the coating chamber 22. One approach to usingthe barrier 36 is to determine the “t₁” and “t₂” time periods for agiven ingot composition, and then programming the apparatus 20 towithdraw the barrier 36 at t₁ following startup of the coating process.The barrier 36 can be later reinserted or the evaporation process simplyterminated at the end of t₂ before evaporation occurs of the YSZ-richremainder of the ingot 10. Alternatively, the operation of the apparatuscould be automated based on sensing the chemistry of the vapor cloud 34.

While the use of a physical barrier 36 is a particularly effectivetechnique for limiting deposition to the intermediate phase (t₂) of thecoating process, other techniques are possible. For example, depositionof a coating rich in the higher vapor pressure constituent(s) of theingot 10 can be avoided by performing the initial phase (t₁) of thecoating process as a separate run, during which the component 30 has notyet been placed in the chamber 22. Deposition of a coating rich in thelower vapor pressure constituent(s) of the ingot 10 can be avoided byterminating the coating process prior to entering the final phase (t₃),i.e., before evaporation occurs of the final portion of the ingot 10that is rich in the lower vapor pressure constituent(s). Furthermore,the latter phase (t₃) of the coating process can be effectivelypostponed as long as ingot material is continuously fed into the chamber22.

In a second investigation leading to this invention, TBC's weredeposited by EBPVD on specimens formed of the superalloy René N5 onwhich a platinum aluminide (PtAl) diffusion bond coat had beendeposited. The specimens were coated by evaporating an ingot of zirconiastabilized by about seven weight percent yttria (7% YSZ) alloyed withabout 16 weight percent ceria. The specimens were loaded into a coatingchamber so as to be supported above the ingot, and the chamber evacuatedto achieve a partial vacuum of about 10⁻⁴ Torr (about 1.3×10⁻⁴ mbar).The specimens were then heated to a temperature of about 900° C. Whilerotating the specimens at a rate of about 25 rpm, the ingot wasevaporated using an electron beam gun operated at a constant power levelof about 24.5 kW. During an initial period of about 10 minutes, thevapor cloud produced by the evaporation process was prevented fromcontacting and condensing on the component with a barrier of the typerepresented in FIG. 1. Coating was then permitted for a duration ofabout 40 minutes by removing the barrier, after which the barrier wasreintroduced to again prevent deposition on the specimens. A TBCdeposited under these conditions is shown in FIG. 3 to have a desirablecolumnar microstructure, while FIG. 4 evidences that the elementaldistribution throughout the thickness of the TBC was substantiallyuniform. FIG. 4 shows that, relative to their stabilized levels in thebulk of the TBC, the zirconium level was relatively high and the ceriumlevel relatively low in the first several micrometers of the TBC. Thecause of this variance is not understood, and in any event would nothave a detrimental effect on the desired properties for the TBC.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. For example, instead of depositing the TBC by EBPVD,other vapor deposition processes could be used. Accordingly, the scopeof the invention is to be limited only by the following claims.

1. An apparatus for depositing a ceramic coating on a surface of acomponent, the apparatus comprising: a single evaporation sourcecontaining multiple different oxide compounds, at least one of the oxidecompounds having a vapor pressure that is higher than the remainingoxide compounds; means for introducing the evaporation source into acoating chamber that suspends the component near the evaporation source;means for projecting a high-energy beam on the evaporation source tomelt the evaporation source, form a molten pool of the oxide compoundsof the evaporation source, and form a vapor cloud having a compositioncomprising the oxide compounds of the evaporation source; a maneuverablebarrier that is maneuverable between a first position in which thebarrier is between the component and the molten pool and a secondposition in which the barrier is withdrawn from between the componentand the molten pool, wherein in the first position the barrier preventsthe vapor cloud from contacting and condensing on the component andwherein in the second position the barrier allows the vapor cloud tocontact and condense on the component to form the coating; means forsensing a chemistry of the composition of the vapor cloud; andautomation responsive to changes in the chemistry of the composition ofthe vapor cloud sensed by the sensing means, the automation positioningthe barrier in the first position if the chemistry of the composition ofthe vapor cloud is different from the evaporation source.
 2. (canceled)3. An apparatus according to claim 1, wherein the at least one oxidecompound is selected from the group consisting of ceria, magnesia,strontia, barium oxide, lanthana, neodymia, gadolinium oxide, dysprosia,ytterbia and tantala.
 4. An apparatus according to claim 3, wherein theevaporation source consists essentially of yttria, zirconia and the atleast one oxide compound.
 5. An apparatus according to claim 4, whereinthe at least one oxide compound is ceria.
 6. An apparatus according toclaim 1, wherein the evaporation source consists essentially of yttria,zirconia and the at least one oxide compound.
 7. An apparatus accordingto claim 1, wherein the at least one oxide compound is ceria.
 8. Anapparatus according to claim 1, wherein the evaporation source containsabout 10 to about 20 weight percent ceria, the balance essentiallyzirconia stabilized by about 3 to about 8 weight percent yttria. 9.(canceled)
 10. An apparatus according to claim 1, wherein the automationpositions the barrier in the second position if the chemistry of thecomposition of the vapor cloud is approximately equal to the evaporationsource.